1. Field of the Invention
The present invention relates to a magnetic toner for use in electrophotography, an image forming method for visualizing an electrostatic latent image and toner jet.
2. Description of the Related Art
In a general electrophotographic method known in the art, an electrical latent image is formed on an image carrier (photoreceptor) by various units using a photoconductive substance; and subsequently, toner is supplied to the latent image to visualize it as a toner image. The toner image is, if necessary, transferred onto a transfer material such as a paper sheet and then fixed the toner image onto the transfer material by application of heat/pressure to obtain a transcript.
As a fixing apparatus used in the fixing step, conventionally various types of fixing apparatuses have been developed. In view of reducing wait time and power-saving, an on-demand system fixing apparatus using a ceramic heater small in heat capacity in combination with a film has been put into practical use.
However, in the on-demand system, an electric field is likely to be generated by frictional electrification between a transfer material, a fixing film and a fixing member such as a fixing roller to attract toner placed on the transfer material to the fixing member. As a result, a part of the toner particles is transferred onto the fixing member and disturbs or smudges the toner image on the transfer material. This is called an electrostatic offset phenomenon. Particularly with an increase of printing speed in recent years, frictional electrification of the fixing member tends to be accelerated. With the tendency, a countermeasure against electrostatic offset induced by a high-speed operation must be urgently taken.
As a countermeasure against the electrostatic offset, forcibly generating an electric field by which electrostatic offset between fixing members is prevented has been proposed. However, an electric field forcibly generated induces local generation of a high electric field on the surface of the fixing member and causes discharge, with the result that a damage such as a pin hole is given to the surface of the fixing member. Furthermore, since a unit for generating an electric field is provided to a fixing apparatus, the structure of a main body is complicated or the fixing apparatus is enlarged. This structural arrangement results in being against miniaturization of a main body in view of save-energy and save-space desired in recent years.
In the circumstances, as a countermeasure against electrostatic offset, many proposals directed to not the main body but toner have been made.
For example, Japanese Patent Application Laid-Open No. 2003-280254 proposes an approach for preventing electrostatic offset by adding a plurality of fine inorganic powders different in resistance to a toner particle and defining the liberation rates of the fine inorganic powders to reduce toner electrification characteristics. However, electrostatic offset can be suppressed to some extent by this method; however, part of liberated fine inorganic powder is often scattered on a transfer material and causes an image defect such as white omission of an image (specifically occurs in a high-speed machine). In addition, if the toner reduced in electrification characteristics in this manner is fixed, since electrostatic adhesion between a transfer material and the toner is low, the toner partly remains on a fixing member without removing from the fixing member. As a result, an image omission occurs, causing image irregularity.
In the circumstances, Japanese Patent Application Laid-Open No. 2005-265958 proposes a solution to these problems by improving not additives but a main substance, i.e., a magnetic toner particle. In this proposal, electrostatic offset is prevented by defining the dielectric constant at 40° C. and a frequency dependence of a dielectric tangent (tan δ). However, the approach may not be sufficient in a recent high-speed fixing apparatus. Furthermore, although the dielectric property at a temperature (40° C.) before a magnetic particle is fed to a fixing apparatus is discussed, dielectric property at a high temperature, i.e., at high-temperature operation during a fixing operation, is not discussed. Thus, in view of properties after thermofusion such as fixation property and mold-releasing property from the surface of a fixing member, a further improvement is required.
As described above, there are a great many technical problems for attaining high image quality free of scattering and image omission; at the same time, for preventing electrostatic offset at the time of high-speed development. A further improvement is required.
An object of the present invention is to provide a magnetic toner in which the aforementioned problems have been overcome. More specifically, the object of the present invention is to provide a magnetic toner providing a high-quality image free of scattering and image omission and capable of preventing an electrostatic offset in a high-speed development system.